For a current radio communication system of a multi-antenna based on OFDM (Orthogonal Frequency Division Multiplexing), e.g., for LTE-A (Long Term Evolution-Advanced) that is being designed, DL joint transmission is an important technique among downlink multi-base station coordination technique. DL joint transmission is to coordinately transmit the same data stream from a plurality of base stations to a terminal, and each base station is directed to terminals through a specific beam. To achieve optimal system performance, in DL joint transmission, there exists a problem how power should optimally be allocated to the base stations. As an optimal power allocating method of this kind, it is generally required that a terminal can measure received power from each base station, and the measured received power can be fed back to a serving base station as grounds of optimal power allocation. FIG. 1 shows a framework of a typical system of DL joint transmission. As shown in FIG. 1, the same data is transmitted from base stations BS 1 and BS 2, and the data reaches a terminal through beam forming. The terminal feeds back the received power to the serving base station (BS 1) and then, the serving base station determines suitable power allocating methods P1 and P2 (assume that total transmitting power is constant: P1+P2=P).
In a multi-antenna system based on OFDM, two kinds of reference signals exist. One of them is a channel state information reference signal (CSI-RS) and the other one is a demodulation reference signal (DM-RS).
The CSI-RS is cell-specific and is used to represent a channel state of a cell in which a user apparatus is not subject to precoding processing. When data is transmitted to a terminal using a resource block in which base stations of cells have the same time frequency resources, at least two CSI-RSs of the corresponding cell are installed in each resource block. To adopt DL multi-base station coordination, for a particular cell, data transmission is punctured in a position corresponding to a CSI-RS of a cell that is adjacent to the particular cell, i.e., no data is transmitted on a resource corresponding to the CSI-RS of the adjacent cell. As a result imposed by such a puncturing technique (because no data is transmitted in the puncturing position), redundant overhead is increased in the system but since the CSI-RS of adjacent cell strictly keeps orthogonality, the accuracy of channel estimation is enhanced for multi-base station coordination.
Unlike the CSI-RS, the DM-RS is UE-specific, and its acting is used for demodulation of the terminal. Since the DM-RS is for a concrete terminal, it is possible for the terminal to detect the received power based on the DM-RS. In DL joint transmission, however, since the terminal uses coherent detection, a signal detected by the terminal is a superimposed signal from a plurality of base stations, the DM-RSs from different transmission base stations include completely the same mode and thus, the DM-RSs of the different transmission base stations are also superimposed on each other. The superimposed DM-RSs are appropriate concerning the coherent detection. The terminal can measure only total received power from a plurality of base stations by the superimposed DM-RS, but the terminal cannot measure received power from a single base station.